The present invention relates to a recording apparatus, and more particularly, to an ink jet recording apparatus.
Conventionally, as described in Japanese Patent Application Laid-open No. 2005-34999, there is disclosed a system which maintains a constant pressure by providing a pressure reducing pump and an atmosphere communication valve to a sub tank, sensing a pressure in the sub tank with a pressure sensor, and driving the pressure reducing pump or driving the atmosphere communication valve based on the pressure value. In addition, an ink jet printer has ink jet heads for individual ink colors, so that sub tanks are provided for individual colors.
However, in the conventional technique, it is necessary to provide pressure reducing pumps that are driven independently to the individual sub tanks. Therefore, the number of components increase, and the system is upsized. In addition, the ink jet head, the sub tank, the pressure reducing pump, the pressure sensor, and the like are mounted on a carriage, resulting in an increase of weight of the carriage. Therefore, a drive motor for driving the carriage is required to be upsized. The increase of the number of components and the upsizing of the drive motor cause an increase of cost. Further, because of contact between ink and air, dissolving of air containing oxygen and nitrogen in the ink occurs, which causes a rapid decrease of pressure accompanying air bubbles when an abrupt flow of ink occurs during a printing operation or a cleaning operation. The air bubbles generate pressure relaxation effect of the ink jet heads and cause ink discharge failure. In addition, if the ink temperature is raised by an increase of temperature of the ink jet heads, acceptable gas dissolving amount in the ink is decreased. Therefore, air bubbles are apt to be generated.
On the other hand, a sub tank having sealed structure has been devised, in which a tension force of a spring or a weight is utilized so that a negative pressure is generated inside. Further description is given with reference to FIG. 5. FIG. 5 is a schematic diagram of an ink container of a conventional sub tank. Two sheets are welded at the periphery thereof so as to have an inlet 9 and an outlet 10 for ink. In order to generate a negative pressure in the ink container 2, a biasing portion 37 uses a tension force of a spring, a weight, or the like for biasing. The internal pressure can be changed by expanding or compressing the ink container 2. In this conventional sub tank, a material and a hardness of the ink container 2 of the sub tank forming the sealed structure largely affect the generated internal pressure. In other words, in order to enhance a gas barrier property, it is necessary to form the ink container 2 of the sub tank by using sheets of structure having both ink resistance and gas barrier property. Therefore, multilayered structure made of plastic thin films or aluminum foil is adopted, and elastic sheets are used. However, as the sheets become harder, a change of pressure increases due to a volume change of the sheets. Thus, it becomes difficult to generate an appropriate back pressure with respect to the ink jet heads. Naturally, the sheet materials for the ink container 2 of the sub tank are restricted, and the softest material among them is selected. However, as the material becomes softer, the ink container 2 is apt to change its volume and has little change of pressure while having poor ability to maintain the shape. If the ink container 2 has once irregular creases 38 and 39, it cannot be restored so that stability of the pressure and repeatability of deformation cannot be maintained. In addition, there is another problem that a fluctuation of pressure among individual ink containers 2 of the sub tank is large.
The reason for those problems is that as the ink container 2 of the sub tank is being filled with ink and is being expanded, the irregular creases 38 and 39 are generated partially at the periphery of the ink container of the sub tank so as to generate a distortion at the periphery, and the distortion causes a plastic deformation. Thus, stability of the operation repeating compression and expansion of the ink container 2 of the sub tank is inhibited when the ink flows in or out of the ink container 2 of the sub tank. Further, there is a fluctuation of deformation of the ink container 2 of the sub tank, which causes a pressure fluctuation among individual ink containers 2 of the sub tank.